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Muscle Contraction Electrical

Experiment 5: Muscle Structure and Function

Introduction

Muscles are found almost any parts of our body. They can be classified in to 2 types (smooth muscle and striated muscle) based on their morphology and functions. The function unit of muscle cells is called sarcomere. The mechanism of all muscles' contraction is generally the same that is based on the sliding-filament theory involving the interaction of the contractile proteins actin and myosin.

Muscles are very important in locomotion, eating process, sound production and etc. different types and patterns of electrical signals will result in different muscle contraction.

The aim of this experiment is to study the muscle contractions when applying electrical signal to a nervous system. The sciatic nerve of the frog was electrically stimulated to mimic the physiological trains of action potentials traveling down the nerve and its innervated muscle. When sciatic nerve was stimulated electrically, it depolarized and generates an action potential which passing along the nerve to gastrocnemius muscle through neuromuscular junction. Gastrocnemius muscle depolarized by the incoming electrical signal and contracted in respond. As a result, the gastrocnemius muscle that located on the upper legs of frog will contract. By adjusting the intensity and frequency of stimulation, the contraction pattern of a single twitch, temporal summation, tetanus and fatigue of muscle were examined.

Procedure

Please refer to the lab manual P.54- P.62, except

  • In part C, students are not required to do single twitch.

2. P.62 Part D Step 8 = stimulate the muscle with electrode (not the clamp)

Precaution

1. Don't touch the nerve with metal (use the glass rod instead), otherwise, no electrical current

  • Rinse nerve and muscle with Ringer water regularly to keep them alive.
  • Leave enough muscle for femur clamp to hold the sample.
  • Don't damage the nerve.

Result

Part A: Single muscle twitch

Test trial

1

2

Intensity of stimulation (mA)

1.5

3

Frequency (Hz)

0.1892

0.1892

Amplitude (mV)

10.73

13.54

Contraction periods (s)

0.309

0.332

Relaxation periods (s)

0.55

0.575

Latency periods (s)

0.041

0.043

Table 1:it shows a single muscle twitch formed under stimulation of different intensities with the same frequency( 2 cases)

Part B: Temporal summation

Intensity of stimulation (mA)

2

Frequency (Hz)

2.038

Table 2: it shows a Temporal summation formed at 2mA and 2.038Hz

Part C: Tetanus

Intensity of stimulation (mA)

2

Frequency (Hz)

5.9277

Table 3: it shows tetanus formed at high frequency of stimulation

Part D: Fatigue - Stimulating the sciatic nerve

Site of stimulation

Sciatic Nerve

Gastrocnemius Muscle Directly

Intensity of stimulation (mA)

2

7

Frequency (Hz)

20

20

Maximum strength (mV)

357.4

208.3

Strength after fatigue 13 seconds after the stimulation (mV)

285.92

166.64

Time to reach maximum (s)

0.95

1.75

Table 4: it shows Fatigue twitches obtained by stimulating sciatic nerve and muscle directly

Discussion

The contraction of muscle is initiated by the propagation of action potential. The muscle membrane depolarizes and will contract. The central nervous system generates action potential which pass through the sciatic nerve and cross the neuromuscular junctions and eventually the muscle will contract.

The central nervous system sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. The action potential is transmitted to the nerve terminal (neuromuscular junctions), the vesicles inside the synaptic knob fuse with the neural membrane and the neurotransmitter acetylcholine that inside the vesicles is released into the synaptic trough. The acetylcholine will then diffuse through the synaptic cleft and is detected by the postsynaptic receptors on the innervated muscles membrane. The acetylcholine-gated ion channels in the postsynaptic membrane open and calcium ions are released into the myoplasm. This will depolarize the muscle membrane. When the reversal potential arrived is greater than the threshold value, it can trigger an all-or-none action potential. It then propagates away from the end-plate in all directions, exciting the entire sarcolemma The muscle will contract based on the sliding filament theory.

Part A: Single muscle twitch

A single threshold stimulus delivered to a muscle. According to graph 1, a single muscle twitch is composed of 3 phases:

  • Latent period for stimulating contraction due to time delay of nervous transmission.

2) Contraction period for shortening muscle.

3) Relaxation period for lengthening muscle.

In this part, 2 conditions of muscle contraction were investigated. At the same frequency of the electrical stimulation, different intensity of stimulation was applied. The latent period of 2 triads is on average 0.042s.

  • Did a more intense stimulation cause a more intense contraction?

Ans: Yes. A more intense stimulation causes a more intense contraction. The amplitude changes from 10.73 to 13.54mV when the amplitude of stimulation increases from 1.5 to 3.0 mA at 0.1892Hz.

  • Why or why not?

Ans: When the action potential passes along the nerve fibers, such current transmits across the neuromuscular junction to muscle cell and causes a depolarization of sacromere. The degree with which tension can be regulated in an “all-or-none” muscle fiber is very small, that is there is no gradation between inactivity and a twitch response. However, some may show stronger muscle contraction when applying stimulation of higher intensity. It is possible when a more motor units are recruited, resulting in increasing number of motor units contraction and hence strength of contraction.

Part B: Temporal summation

When the muscle is stimulated several times in a row, the intensity of the contractions within each twitch will increase. This effect is called temporal summation. When the frequency of the stimulation is low, adequate time is allowed for the muscle to relax completely before the second stimulation. However, when the frequency is sufficiently high, no time is given for the muscle cells to relax completely. A rapid succession of stimulating impulse will result in stronger contraction. It is because the second depolarization adds the first one before its refractory period, a temporal summation of contractions occurs.

  • How can the muscle show more contraction on a second, or later, a twitch?

Ans: the muscle can show more contraction on a second when the frequency of stimulation is high enough. In this part, we generate a temporal summation by using 2.038Hz of stimulation.

The first action potential arrived opens voltage-dependent Ca2+ channels in the muscle membrane. The concentration of intracellular free Ca2+ ions in muscle cells is then increased for a short time. When the second impulse arrives at the terminal where the intracellular Ca2+ concentration is still somewhat elevated, the amount of Ca2+ ions entry resulting from the second action potential add to the remaining Ca2+ to generate an even higher concentration. The addition of Ca2+ concentration produces a large increase in the amount of transmitter released subsequently to the second impulse. When the frequency of stimulation is high, time is not enough for the removal of calcium ions from the troponins. As a result, the tropomyosin threads still stay into the grooves between the two F-actin filaments. The actins binding sites are exposed and the contraction of muscle can still occur. Therefore, a twitch of higher amplitude in further summation can be achieved in such rapid succession theoretically. This is the effect of temporal summation.

  • Do you observe temporal summation? Make a hard copy of your results.

Ans: Yes, I did. Temporal summation can be observed in graph 3.

Part C: Tetanus

Tetanus is formed when the stimulation of a muscle is prolonged. The stimuli are so close together that the muscle as a whole cannot get enough time to relax between twitches. The muscles remain contracted. This results in a smooth, sustained contraction and is called tetanus.

During muscle contraction, contractile elements are being physically stretched. These elements will then store potential energy in a series of elastic elements. If the propagation of action potentials is very fast, the Ca2+ ions are not completely removed from the myoplasm into the sarcoplasmic reticulum, the concentration of Ca2+ remains high in myoplasm and the muscle tension keeps prolonged. No relaxation period of muscle occur. The Twitches that induced by the rapid succession of stimulation fuse together and tetanus tension would be reached when applying a maximum frequency to the muscle.

  • Does the muscle show temporal summation at first?

Ans: Yes, the muscle shows a temporal summation at first. In fact, tetanus is a kind of temporal summation when the frequency of stimulation from the nerve is very high. The temporal summation appears first because the sarcomere cannot reach its minimal length at the first single stimulation. It needs many continuous fast stimulations in order to achieve tetanus. It only showed a smooth and sustained contraction called tetanus which resulting from sufficiently rapid succession of stimulation later. The process is shown in graph 4.

  • Does the strength of contraction during tetanus gradually decrease during the sustained contraction?

Ans: No, it doesn't. The strength of contraction kept more or less the same during sustained contraction.

  • When might a tetanic contraction occur in your own body?

Ans: titanic contraction often occurs when we are doing heavy exercise or processing isometric contraction such as picking up a heavy load for a prolonged time. At that time, our nervous system need to keep sending action potential at a high frequency to keep the posture of our body and to keep our body muscle contracted without relaxation. The tension within the contractile elements continues to increase with time. When the stretching of series elastic component is just sufficient to prevent further shortening of muscle, the muscle has then reached the full tetanus tension.

Part D: Fatigue

Fatigue occurs in muscle after prolonged strong contraction with comparably high frequency stimulation. Muscle fatigue is the decline in ability of a muscle to generate force. a depletion of oxygen and energy sources in muscle, the limited synthesis of ATP and the Accumulation of lactic acid in muscle are factors causing muscle fatigue.

8) Was the decrease in the strength of contraction gradual or sudden?

Ans: The strength of contraction decreases gradually as seen in graph 5.

In this experiment, the muscle of frog was tested. Since the frog muscle was cut away from its body, there is no blood supply to the frog muscle being examined. However, the oxygen and ATP within the muscle were continuously used up and the lactic acid was continuously built up. Without enough energy sources, the muscle cannot contract so well. The accumulation of lactic acid decreases the sensitivity of the nerve impulse for muscle contraction. The strength of muscle contraction after fatigue declines gradually as the lost of energy source and the accumulation of lactic acid occur gradually.

9) Was the muscle itself fatigued?

Ans: Yes, it was. The muscle was stimulated directly by inserting a stimulating electrode into it. According to graph 6, a gradual decrease in the intensity of muscle contraction was shown, which indicate the muscle itself is also fatigue as well as the fatigues in neuromuscular junction. The strength of contraction generated by nerve stimulation is much larger than those by muscle stimulation. This is because the nerve innervate deeply and widely in the neuromuscular junction, which causes a rapid transmission of action potential to the entire muscle instead of locally by direct stimulation of muscle. As a result, more motor units are activated and involved so that a more intense contraction occurs.

10) What could cause fatigue at the neuromuscular junction?

Ans: the neuromuscular junction is used to transmit signals from the nerve to stimulate the muscle. When the signal arrives at the end of the nervous terminal, the neurotransmitter acetylcholine is released from the vesicles and is used to transmit the signal to the postsynaptic membrane. Acetylcholine will then be broken down in the postsynaptic membrane. The fatigue at the neuromuscular junction could be due to a slow production of neurotransmitters acetylcholine in the presynaptic vesicles, which cannot compensate its consumption. When the supply of neurotransmitters is in shortage after heavy exercise, the muscle starts to fatigue.

11) Does the muscle eventually show fatigue at this rate of stimulation? Make a hard copy of your results.

Ans: Yes, the muscle is eventually fatigued

Conclusion:

In this experiment, it can be concluded that the nerve controls the contraction and relaxation of the muscle. By applying different frequency and intensity of the electrical current, different status of muscle can be observed. Temporal summation occurs when a high frequency of stimulation is applied for a short time. Tetanus occurs when high frequency stimulation is prolonged applied. Fatigues occur after prolonged strong stimulation is applied which results in reduced strength of contraction.


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